1. Field of the Invention
The present invention relates to a technique of supplying lubricating oil using an oil supply unit that has a pump that operates through drive of a piezo element.
2. Description of the Related Art
In recent years, various types of machine tools are requested to increase the speed of a main spindle in order to improve the processing efficiency and the production efficiency. When the main spindle is rotated at a high speed, the lubricating property of a bearing portion that supports the main spindle is particularly important. Thus, there is proposed a rolling bearing device in which an oil supply unit is provided adjacent to a bearing portion in the axial direction (see Japanese Patent Application Publication No. 2016-23759 (JP 2016-23759 A)). The oil supply unit has a tank that stores lubricating oil, a pump that discharges the lubricating oil in the tank to the bearing portion, etc.
The oil supply unit described above is installed, together with the bearing portion, in a narrow annular space between the main spindle and a housing on the outer side thereof, for example. Therefore, with the oil supply unit, it is preferable to reduce the frequency of replenishing the tank with lubricating oil as much as possible in order to improve maintainability and manageability. To this end, it is necessary to suppress wasteful discharge (consumption) of lubricating oil by the pump, and the pump shall preferably discharge only a minute amount of lubricating oil.
In order to discharge only a minute amount of lubricating oil, there is proposed a pump (piezo pump) that discharges lubricating oil through drive of a piezo element. In the case of this pump, however, the velocity and the amount of lubricating oil (oil droplets) to be discharged are varied in accordance with the viscosity of the lubricating oil. The viscosity of lubricating oil is significantly affected by the temperature thereof. That is, in the case where the viscosity of lubricating oil is high (the temperature is low), the discharge velocity is low, or lubricating oil may not be discharged. In the case where the viscosity of lubricating oil is low (the temperature is high), in contrast, the discharge velocity is high, and the amount of discharge tends to be large, which may lead to wasteful consumption. In the case of a piezo pump, in this way, the manner of discharge of lubricating oil may be fluctuated in accordance with the viscosity (temperature).